Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
During the last couple of years the following trends have however become apparent:                UE capabilities and processing power have increased considerably. This is driven both by the development of the long-term evolution (LTE) in order for supporting high peak data rates and the multi-carrier (MC) evolution within WCDMA/HSPA.        The user demand for high peak data rates and operators' desire to manage their wireless resources efficiently have (and continue to) increased. This is a consequence of that mobile operators have started to rely on WCDMA/HSPA/LTE technology to offering mobile broadband services.        
FIG. 1 shows one inter-site multi-point transmission structure, which includes the first radio network node (“1st Node” for short) and a number of second radio networks nodes (“Second Node 1”, . . . , “Second Node N” for short). Each second radio network node or point transmits different data blocks to the same User Equipment (UE). The first node and the second nodes are logical nodes, which can physically be standalone nodes or part of other network nodes. They can even be co-located in the same site or can be located separately. The first node can be the controlling node, which controls and manages the second nodes. The second nodes can be the radio node equipped with radio transmitter and receiver. Moreover, a second node also has functionality to schedule the data for UEs served by this node. The first node splits the data among different second nodes which in turn eventually transmit it to the UE. Thus each second node schedules and transmits the data received from the first node to the same UE. On the UE side, the data blocks received from all the second nodes are separated and reordered according to special services.
The general architecture shown in FIG. 1 is applicable to any radio access technology using multi-point transmission. For example, the structure shown in FIG. 1 is applicable to HSPA, LTE, GERAN, CDMA2000, or any combination of RATs. As another example, the general architecture is also applicable to multi-RAT multi-point transmission e.g. LTE+HSPA.
A third network node is any node which can be connected to the first and/or second node. But third node is not directly involved in multi-point transmission. Examples of third node include OSS, SON, MDT, and the like.
MP-HSDPA is one example of the inter-site multi-point transmission illustrated in FIG. 1. These observations have triggered discussions in the Third Generation Partnership Project (3GPP) on standardizing support for multi-cell transmissions to improve the downlink experience for UEs at the cell edge. In case of HSPA, the 3GPP technical report is available for MP-HSDPA (Reference [1]). In the following, the MP-HSDPA is briefly introduced.
The multi-point transmission for LTE was investigated in (Reference [2]). It is being currently standardized for LTE in Rel-11.
The terms, “coordinated multi-point (COMP) operation” and “multi-point transmission” are interchangeably used with the same meaning in the present disclosure.
The multi-cell transmissions techniques in HSDPA discussed during 2010 include:                1. Switched Transmit Diversity Techniques (e.g., HS-DDTx or SF-DC-HSDPA switching): This class of techniques is based on that HSDPA transmissions from different cells are coordinated so that the inter-cell interference is minimized; thereby a virtual (“soft”) reuse factor is introduced.        2. Multi-Flow Transmission Techniques (e.g. SF-DC-HSDPA aggregation): This class of techniques is based on that several, independent data streams are transmitted to the same UE from different cells located in the same or different sites. The gains associated with this class of techniques stems from “spatial resource pooling”.        3. Single Frequency Network Transmissions (e.g. HS-SFN): This technique is based on that identical data to the same UE from multiple cells simultaneously. This technique is based on that the transmitted data is combined in the “air” and the UE will consequently benefit from a stronger received signal.        
The multi-cell transmission can also be multi-frequency multi-cell HSDPA (MF-MC-HSDPA). A special case of MF-MC-HSDPA is DF-DC-HSDPA in which two independent data streams are transmitted over two different carrier frequencies to the same UE from the cells located in the same or different sites.
FIG. 2 illustrates the MP-HSDPA structure, where the first node is RNC and the second nodes are two Node Bs. MP-HSDPA is being currently standardized in Rel-11. SF-DC-HSDPA is the most promising among these possible choices. There are two modes in the SF-DC-HSDPA, SF-DC Aggregation and SF-DC Switching.                SF-DC Aggregation                    Either or both of dual cells can simultaneously transmit different transport blocks to the same UE.            The two cells can belong to the same NodeB (Intra-NodeB aggregation) or different NodeBs (Inter-NodeB aggregation).            The SF-DC UE with advanced receiver can gain from this approach by suppressing interference between flows.                        SF-DC Switching                    Only one of dual cells can transmit data block to the UE at a certain TTI.            Better cell is selected to transmit data (e.g. a cell with higher CQI).            The H-ARQ retransmission to a UE can be scheduled in either cell.            The UE with less advanced receiver can also gain from this approach.                        
The SF-DC in 3GPP is already extended to DF-DC/DF-4C, which is similar but with more frequencies and more cells.
For a UE in SF-DC-HSDPA mode, the network should distribute the data proportionally to the achievable rates in two different cells in order to ensure the in-sequence delivery of RLC layer packets on the UE side.
In case of the intra-site SF-DC HSDPA, the data can be easily split by the Node B at the MAC-ehs layer according to the scheduled radio resources. This means the data splitting at the MAC-ehs layer can match the achievable data rate in the air interface pretty well in both the HS-DSCH serving cell and the second HS-DSCH serving cell. The in-sequence delivery of RLC layer packets is not deteriorated. FIG. 3 illustrates the data split in the Node B on MAC-ehs level.
In case of the inter-site SF-DC, the data has to be split by the RNC. There are several proposals to split the data to 2 sites. Two typical examples are listed below.                1. Data Splitting On RLC Level                    According to this method, the RNC splits the data for the 2 sites in RLC layer according to the corresponding data rates that can be guaranteed by these 2 sites. FIG. 4a illustrates the data splitting on the RLC level.                        2. Data Splitting On PDCP Level                    According to this method, the RLC layer splits the RLC PDUs for transmission by the 2 sites according to the corresponding data rates that can be guaranteed by these 2 sites. FIG. 4b illustrates the data splitting on the PDCP level.                        
The scheduling priorities of SF-DC-HSDPA UEs are typically lower than the non-SF-DC-HSDPA UEs, due to that the SF-DC-HSDPA UEs suffer from bad channel quality. Moreover, due to interference between the two simultaneous data streams from the two HS-DSCH serving cells, a SF-DC-HSDPA UE statistically has low radio resource utilization efficiency compared to non-SF-DC-HSDPA UE. According to the assumptions used in 3GPP simulations, the SF-DC-HSDPA UEs can be scheduled in a HS-DSCH service cell only when there are still radio resources left after all other schedulable UEs are scheduled. The achievable data rate in a HS-DSCH serving cell for a SF-DC-HSDPA UE relies not only on the channel variation of this SF-DC UE but also many other factors in the HS-DSCH serving cell: the downlink traffic load change, the radio channel variation of other UEs, the inter-cell interference variation. Hence for a SF-DC-HSDPA UE, the achievable data rate in a HS-DSCH serving cell can quickly change with large variation.
In LTE the multi-point transmission, which is more commonly called as CoMP is similar to that in HSDPA. However different terminologies are used in LTE and thus CoMP or multi-point transmission in LTE may be categorized into one of the following categories:                Joint Processing (JP): Data for a UE is available at more than one point in the CoMP cooperating set (definition below) for a time-frequency resource. To achieve this simultaneous data transmission from multiple points (part of or entire CoMP cooperating set) to a single UE or multiple UEs in a time-frequency resource (e.g. resource blocks) takes place. In one scenario the data to a UE is simultaneously transmitted from multiple points, e.g. to (coherently or non-coherently) improve the received signal quality and/or data throughput. In another scenario the data transmission from one point (within the CoMP cooperating set) in a time-frequency resource. The transmitting/muting point may change from one subframe to another including varying over the RB pairs within a subframe. Data is available simultaneously at multiple points.        Coordinated Scheduling/Beamforming (CS/CB): Data for an UE is only available at and transmitted from one point in the CoMP cooperating set (DL data transmission is done from that point) for a time-frequency resource but user scheduling/beamforming decisions are made with coordination among points corresponding to the CoMP cooperating set. The transmitting points are chosen semi-statically.        Hybrid category of JP and CS/CB may be possible: Data for a UE may be available only in a subset of points in the CoMP cooperating set for a time-frequency resource but user scheduling/beamforming decisions are made with coordination among points corresponding to the CoMP cooperating set. For example, some points in the cooperating set may transmit data to the target UE according to JP while other points in the cooperating set may perform CS/CB.        
In LTE the handling of higher layer packet or more specifically the splitting of higher layer packets (e.g. RLC, PDPC level packets etc) for joint processing or coordinated scheduling/beamforming combination thereof can be performed in a suitable network node. The example of suitable network node which can perform such functions is one of the eNode B involved in the CoMP, a separate radio network node controlling or managing nodes involved in CoMP. Such eNode B or any controlling network node may be termed as a primary node or primary eNode B, a master node or a master eNode B.
For the sake of simplicity a generic term ‘coordinated scheduling of a downlink transmission flow’ is used in the disclosure. However it should be noted that such a term includes all different types of multipoint transmission schemes in HSPA, LTE and also in other technologies including multi-RAT multipoint transmission scenario.